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Kinetics and Mechanism of Oxidation of Hydroxyurea with Hexacyanoferrate( III) Ions in Aqueous Solution

Hydroxyurea (HU) effectively reduces Fe(CN)6 3− to Fe(CN)6 4− species in neutral and basic aqueous solution via an electron transfer process that includes the formation and subsequent fading out of a free radical, U• (U• ≡ H2N−C(=O)N(H)O•). The EPR spectrum of U• in H2O solutions suggests that the unpaired electron is located predominantly on the hydroxamate hydroxyl-oxygen atom. Visible spectrophotometric data reveal HU as a two-electron donor. Stoichiometry of the studied reaction can be formulated as: 2Fe(CN)6 3− + NH2CONHOH + ½H2O  2Fe(CN)6 4− + CO2 + NH3 + ½N2O + 2H+. Lack of evidence for the formation of NO probably is a consequence of fast dimerization of HNO in comparison with the rate of its oxidation, which is slow due to the low reduction potential of the Fe(CN)6 3−/ Fe(CN)6 4− couple. The kinetic of oxidation of HU by Fe(CN)6 3− was studied using stopped-flow technique, as a function of H+, HU, Fe(CN)6 3− and Fe(CN)6 4− concentrations, as well as a function of ionic strength and temperature. The kinetic results reveal that oxidation of HU by Fe(CN)6 3− proceed via an outer-sphere electron-transfer process. The effect of ionic strength on the reaction rate reveals that NaFe(CN)6 2− is the reacting species rather than Fe(CN)6 3− ion. The rate of the redox process was found to be first order with respect to both redox reactants while the H+ concentration dependence make clear that U− is about four orders of magnitude more reactive than HU. The formal reduction potentials for U•/U− and HU•/HU couples were estimated from the kinetic results as +0.47 V and +0.84 V, respectively.

hydroxyurea; hexacyanoferrate(III); kinetics; redox reaction; oxidation; mechanism

Rad je kao poster prezentiran na skupu 28th International Conference on Solution Chemistry. održanom od 23.-28.09.2003., Debrecen, Mađarska ; objavlejn u Knjizi sazetaka ; Lazar, Istvan ; Toth, Imre (ur.) ; 2003 ; str. 130-130.